I've used the OMM 15l (same dsign as the 12, just slightly larger) for a couple of years and reckon it's a great pack. My only gripe is the mesh pockets on the sides can be tricky to get bottles in and out of when the pack's full.
Recently bought a 14l Raidlight pack which has been great so far, better than the OMM I reckon. A bit of extra padding for comfort, more pockets for stashing smaller bits of kit I might want to access frequently and better bottle holding pockets. It's also got a bladder pocket on the exterior, meaning it's much easier to refill the bladder, if you use one.